I usually freese treated water in an ice cube tray. When the temp gets above 80, i put 2-3 cubes in the filter every hour or so. That way i lower the temp gradually. (mind you, i have a 10gal, a bigger one would take lots more ice... lol)
You can use water chiller, but expensive. Air conditionning will help keeping your tank and you happy ;-) If you choose water chiller, big als online has one on sale (44% off) that is good for up to 90 gallons.
Put your heater on the lowest, or even off. You dont whant to raise the PH so get a few water bottle's and freeze them. Then let them float in your tank.
I posted this idea a while back ...id really love to see someone make it...i will eventually but im waiting for the big tank to get done to use it on there ....if you decide to do it, keep us posted on how it works.................
i had a similar problem a while ago--i have the african summer to deal with :)
i merely switched the heaters off for a while, or i turn it down to the minimum temp my fish would be happy with-as the guy above suggested!!!!!
oh-i keep SA cichlids-so they to quite like specific temps!!!
im not sure the conversion to celsius, but malawis should be ok at 28C????
one thing I've been doing lately (thanks to a slight heat wave) is to keep th elights off during the day and the room dark and turn the lights on at night with the windows open. keeps the temp from rising too much during the day. I noticed that the temp is stable all times of the day... for now at least =)
This only happens to mine in the summer. I will fill a Ziploc baggie with ice cubes, and then float it in the tank. One small bag will bring my 20 gallon down by 2 degrees.
my tanks right now are all at about 89*... my room is freaken hot with all of the reptiles and stuff... plus no insulation... i figure that it's kind of natural for the water temp to rise in the summer... like all of the reptile's are up to almost 100* and some are actually at 120* to replicate poor ass africa...
the SW tank i'm starting is at a constant 79* for some reason... and the heaters turn on at 75*... damn i needs to get a chiller...